The present invention generally relates to call signal generating circuits, and more particularly to a call signal generating circuit which generates a call signal by carrying out a switching control.
A call signal which is used to call and ring the bell of the telephone set from the switching system is also sometimes referred to as a ringing signal. For example, a low-frequency signal of 16 Hz and 75 V is turned ON for one second and turned OFF for two seconds to form the call signal. The call signal is generated by a call signal generating circuit, and there is a demand to improve the characteristic and efficiency of the call signal generating circuit. The call signal was originally a sinusoidal wave, but the call signal was replaced by a staircase wave which approximates the crest factor of the sinusoidal wave.
Generally, the voltage supplied from the switching system to the telephone set of the subscriber is 48 V. On the other hand, the call signal has the frequency of 16 Hz and the peak voltage of 75 V. Hence, a step-up transformer is used to boost the voltage when forming the call signal. However, the step-up transformer designed for the low-frequency of approximately 16 Hz is bulky, and the method of boosting the voltage using a high-frequency signal was conventionally used.
FIG. 1 shows an example of a conventional call signal generating circuit. The conventional call signal generating circuit includes a transformer 41 having a primary winding 41a and secondary windings 41b and 41c, a transistor 42, a capacitor 43, diodes 44 and 45, photo-transistors 46 and 47, a capacitor 48, a control circuit 49, photodiodes 50 and 51, a dummy resistor 52 and output terminals 53 which are connected as shown in FIG.1.
A D.C. voltage is applied to the primary winding 41a of the transformer 41 via the transistor 42, and this transistor 42 is turned ON/OFF at a frequency of several tens of kHz to several hundred kHz by the control circuit 49. Accordingly, compared to the case where the 16 Hz signal is boosted, it is possible to reduce the size of the transformer 41. The voltage of several tens of kHz to several hundred kHz induced at the secondary windings 41b and 41c of the transformer 41 due to the ON/OFF control of the transistor 42 is rectified by the diodes 44 and 45. The diode 44 rectifies the voltage into a positive polarity voltage, while the diode 45 rectifies the voltage into a negative polarity voltage. The phototransistors 46 and 47 are respectively turned ON/OFF by the voltages from the diodes 44 and 45, and the phototransistors 46 and 47 are turned ON/OFF alternately. Hence, a positive polarity voltage from the phototransistor 46 and a negative polarity voltage from the phototransistor 47 are output alternately, and a call signal of 16 Hz is output via the output terminals 53.
In this case, a photo coupler is formed by the phototransistor 46 and the photodiode 50, and another photo coupler is formed by the phototransistor 47 and the photodiode 51. The photodiodes 50 and 51 are alternately driven by the control circuit 49 for a time which is shorter than one-half the period of the 16 Hz call signal. Accordingly, the phototransistors 46 and 47 are alternately turned ON with respective quiescent times. The capacitor 48 is provided to eliminate switching frequency components of several tens of kHz to several hundred kHz induced at the secondary windings 41b and 41c, other high-frequency components and the like. The dummy resistor 52 is provided to discharge the capacitor 48.
FIG.2 is a time chart for explaining the operation of the conventional call signal generating circuit shown in FIG. 1. In FIG. 2, (a) shows a current supplied to the photodiode 50, (b) shows a current supplied to the photodiode 51, and (c) and (d) show the call signal output via the output terminals 53. As described above, the photodiodes 50 and 51 receive the currents from the control circuit 49. If the 16 Hz call signal has a period T1, the current is supplied to the photodiode 50 for a time T2, the current is supplied to the photodiode 51 for a time T4 and quiescent times T3 and T5 are provided as shown in FIG. 2, measures are taken so that T1=T2+T3+T4+T5, T2=T4 and T3=T5. In this case, the crest factor of the sinusoidal signal which is equal to (crest value)/(effective value) is 1.414, and the quiescent times T3 and T5 are set so as to approximate this crest factor. The effective value is the average of the square of the instantaneous value of one period of the fundamental wave, and the crest value is the peak value of the wave.
When the currents are supplied to the photodiodes 50 and 51 which form the photo coupler with the respective phototransistors 46 and 47, the phototransistors 46 and 47 are turned ON by the light from the photodiodes 50 and 51 applied to the bases of the phototransistors 46 and 47. The phototransistors 46 and 47 are respectively turned ON during the times T2 and T4 in FIG. 2. As a result, it is possible to obtain the call signal which has the staircase waveform with the crest factor approximating the crest factor of the sinusoidal wave.
If the dummy resistor 52 is omitted and the call signal generating circuit assumes a low-load state, it becomes equivalent to the case where the discharge time constant of the capacitor 48 is large. Hence, the falling edge (and the corresponding rising edge) of the waveform becomes gradual in this case as shown in FIG.2 (d). That is, the waveform shown in FIG. 2 (d) is different from the waveform which is set to have the crest factor approximating the crest factor of the sinusoidal wave and does not meet the specifications. Therefore, the dummy resistor 52 is essential in the conventional call signal generating circuit.
As described above, the 16 Hz call signal is generally used. However, in order to reduce the size of the step-up transformer, the method of boosting the voltage by a high-frequency switching is conventionally used. In this case, the call signal used has the staircase waveform shown in FIG. 2 (c) which has the crest factor approximating the crest factor of the sinusoidal wave. The dummy resistor 52 is provided to maintain this staircase waveform shown in FIG. 2 (c). However, there were problems in that a power loss is introduced at the dummy resistor 52 and that the heat generated from the dummy resistor 52 prevented the effective reduction of the size of the call signal generating circuit.
In other words, the power loss at the dummy resistor 52 is large if the resistance of the dummy resistor 52 is small. However, the discharge of the capacitor 48 becomes insufficient if the resistance of the dummy resistor 52 is made large in order to reduce the power loss. For this reason, it was inevitable to set the resistance of the dummy resistor 52 to the small value in order to ensure sufficient discharge of the capacitor 48, although the unwanted power loss occurred.